Mobile telecommunications networks enable users of User Equipment (UE) to communicate with other such users via one of a number of base stations and a core network. Each base station defines a number of cells of the network. In an active or connected state a UE is registered with the network and has an RRC (Radio Resource Control) connection with a base station so that the network knows to which cell the UE belongs and can transmit data to and receive data from the UE. In the Long Term Evolution (LTE) of UTRAN (UMTS Terrestrial Radio Access Net(work) referred to as E-UTRAN, in the connected state, the handover procedure allows UEs to have service continuity while moving between LTE cells and when moving to cells of other RATs (Radio Access Technologies), such as UTRAN cells and GERAN cells. Prior to handover, the serving cell requests the UE to make measurements on neighbouring cells identified on a cell list and to return the measurement results to the serving cell. The serving cell then uses these measurement results to select the target cell for the handover. This selection is usually based on the services that the candidate cells can provide and the signal quality measurements obtained from the UE.
A UE also has a power conservation or idle state in which, typically, the UE is not transmitting or receiving data and no context about the UE is stored by the base station. In the idle state the location of the UE is known only (to the MME (Mobility Management Entity) in 3GPP) at the granularity of a Tracking Area (TA) comprising a cluster or group of base station cells. When in the idle state, a UE reselects cells on which it will “camp” according to the parameters broadcasted in the BCH (Broadcast Channel) of the current cell on which it is camped, and the base stations are not aware of the cell reselections made by the UE as it moves within the network.
Release 8 of the LTE and UTRAN standards introduced the feature of the base station maintaining history information identifying where the UE has roamed during the connected mode and providing this history information to a target base station at the time of handover. This information is useful for making future handover decisions and for detecting unwanted “ping pong” situations where a UE is repeatedly transferred between two base stations. When in the idle mode, the UE may also maintain history information identifying the cells in which the UE has camped. When the UE enters its connected mode, the UE may then provide this history information to the serving cell. Again this UE generated history information may be useful for future handover decisions.
Under the 3GPP standards, a UTRAN base station is referred to as a NodeB and an E-UTRAN base station is referred to as an eNodeB or eNB. Recently the 3GPP standards body has adopted an official architecture and started work on a new standard for home base stations (NM). Where the home base station is operating in accordance with the LTE (Long Term Evolution) standards, the HNB is sometimes referred to as an HeNB. A similar architecture will also be applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HNB to refer to any such home base station. The HNB will provide radio coverage (for example, 3 G/4 G/WiMAX) within the home and will connect to the core network via a suitable public network (for example via an ADSL link to the Internet) and in the case of the 3GPP standards, via an optional HNB gateway (HNB-GW) which typically will aggregate traffic from several HNBs.
The HNB may be configured to operate using one of a plurality of access modes, namely: ‘closed’ in which the HNB operates as a closed subscriber group (CSG) cell; ‘hybrid’ in which the HNB operates as a CSG cell where at the same time, non-CSG members are allowed access (for example, to allow preferential treatment of CSG members); and ‘open’ in which the HNB operates as a normal (non-CSG) cell. It is understood that when the UE is close to its HNB it is preferable that it attempts to handover to its HNB. However, it is expected that more and more HNBs will be deployed and it will therefore be difficult for the base stations neighbouring the HNB to be able to identify the HNB of a specific UE.